During industrial manufacturing of many types of metal goods, it is common practice to weld together two or more separate pieces of metal to form a unitary structure. Normally, welding is confined to a relatively narrow zone of the total structure formed by welding, but it produces small and very hot, often molten, bits of metal that are discharged into the space surrounding the welding zone and often come into contact with and, unless inhibited from doing so, stick to, nearby areas of the structure being formed by welding. Any such weld splatter adhering to the structure being formed by welding becomes a surface defect, and, if the surface of the structure being welded is intended eventually to become a glossy finished surface, such as an automobile body, file cabinet, or the like, any such defects must be prevented or repaired in order to achieve a satisfactory finish. Repairing defects of this type is relatively expensive, so that prevention is highly preferred.
The adhesion of welding splatter can be inhibited by covering surfaces in the vicinity of welds with an inhibiting composition. Compositions now in commercial use for this purpose are believed to consist primarily of water, alkali neutralized complex acrylic acids, high foaming nonionic surfactants, and defoamers. These materials are often not fully satisfactory for at least two reasons: They have sufficiently high viscosity to develop noticeable thickness variation ("sagging") on non-horizontal surfaces, and they offer insufficient protection against corrosion when surfaces coated with them are stored for more than a day or two.
During the manufacture of automobiles and other metal goods with decorative glossy surfaces, it is common practice to inspect Class A surfaces for metal surface shape irregularities (dents or dings) prior to painting the surfaces; this inspection and/or the technique used in it is commonly referred to as "hiliting" or "highlighting". Detecting and correcting a metal defect prior to painting simplifies the painting operation and precludes the need for subsequent removal of paint to repair the defect and repainting of the surface. Repairing the defect and repainting results in higher costs for time, materials and labor while reducing product quality.
During automotive assembly operations, bare metal panels such as doors, fenders, roofs, quarter panels, and the like are attached to the frame of the vehicle. The vehicle is then cleaned to remove oil and dirt as well as any grinding dust. The clean metal has a dull finish which makes it difficult to see minor dents and dings in the metal panels of the vehicle.
In practice, metal parts to be painted, especially sheet metal parts of an auto body, are first assembled into a subassembly. The subassembly is then cleaned to remove oxidation, oil, and any other contaminants on the surface to be painted. The cleaned subassembly is then coated with the hilite composition and inspected. A hilite material is applied to the vehicle either by hand wiping or by a mechanical device, typically a number of rotating brushes that have been wetted with a hilite solution. Excess hilite solution is permitted to drain for a short period and the resulting glossy surface of the vehicle is inspected for dents and dings. Any dents and dings found are marked for repair and then lightly sanded and/or bumped out as needed.
The subassembly after hilite inspection and repair, if necessary, is then treated to remove the hilite material. A protective coating such as a phosphate conversion coating is applied to the metal and the metal is then coated with a decorative and protective organic coating.
To be optimally useful, a hilite composition must rapidly form a glossy, foam free coating on the metal surface, must not stain or etch the metal, and must be readily removable from the metal.
Frequently, inspection of the partially assembled vehicles is not performed immediately after application of the hilite solution because of floor space restrictions, lunch breaks, line stoppages, weekend down time, or the like. Such delays impose major performance requirements on the hilite composition. During any delay from application of the hilite composition to inspection, the gloss of the treated surface should not change visually due to drying; the retained hilite solution must not etch or stain the metal surface or otherwise make the surface unsuitable for painting; and the retained hilite solution should provide some corrosion protection to the metal. (An unprotected metal surface, particularly one of zinc alloy coated steel, aluminum, or cold rolled steel, will very quickly rust or otherwise oxidize.)
Currently available hilite compositions normally contain solvents such as kerosene or mineral spirits or water soluble solvents such as glycol ethers. The compositions also contain surfactants, typically nonionics, and corrosion inhibitors such as petroleum sulphonates, triethanolamines or sodium nitrite. The commercially available hilite compositions provide a temporarily glossy surface and short term corrosion protection. The solvents used in the presently available formulations are volatile. Kerosene and mineral spirits evaporate much more rapidly than glycol ethers, but even glycol ethers are considered to be VOC for pollution control purposes, so that these compositions must be used in compliance with government regulations and restrictions for VOC use. (See ASTM 2369, "Volatile Content of Coatings".) Avoidance of this problem is one object of the present invention.